The simplest way to add a harmony note to the bass is to select a note from the given chord. For example, in bar 1 the notes of the Am chord are A, C and E, so any of these could be used to harmonise the A bass note. Your choice of note will depend on the overall sound of the various combinations (e.g., in bar 4, the E note is used in preference to the B note). In some cases it may also depend on the ease of playing.
The notes that you are adding in this step of the arrangement can be referred to as a middle voice because they occur between the bass and melody lines.
* Harmony can be defined as the simultaneous sounding of two or more different notes.